24.1 The Distal Radioulnar Joint (The Mobile DRUJ)

The distal radioulnar joint (DRUJ) is an articulation between the ulnar head and the sigmoid notch of the distal radius (▶Fig. 24.1). The DRUJ functions as a pivot joint allowing the radius to rotate around the ulna within a range of movement restricted by soft tissues that connect the radius and the ulna and also those surrounding this joint. The stability of the DRUJ is provided by a combination of cartilage-to-cartilage contact between the sigmoid notch and the ulnar head, and the soft-tissue stabilizers. ¹ The triangular fibrocartilage complex (TFCC), the interosseous membrane of forearm, and the extensor retinaculum work as static stabilizers of the DRUJ. ^{2 , 3 , 4 , 5} The extensor carpi ulnaris (ECU) tendon and the pronator quadratus muscle work as dynamic stabilizers of this joint. ^{6 , 7 , 8} These dynamic stabilizers work independently or cooperatively according to the changing kinematics during forearm rotation.

Comparative anatomical studies of the wrist joint revealed that the mobile DRUJ is one of the major dominant traits inherited in hominoids from the common ancient ancestor of humans and anthropoid apes. This unique anatomical feature might have been advantageous to their life in the trees, especially for locomotive brachiation. ⁹ It is considered that the mobility of the DRUJ, along with the prehensile thumb, are hallmarks of humans, and helped in handling tools, in using weapons to defeat enemies, and improved survival in lethal environments. The mobile DRUJ also helped creativity, with improved upper extremity function being more useful to the imagination of the larger more capable brain that evolved. ¹⁰

The characteristics of the human mobile DRUJ that differentiate it from the animal DRUJ with limited mobility include (1) the formation of diarthrotic DRUJ, (2) the absence of ulnocarpal articulation, and (3) the presence of TFCC. ⁹ In contrast with the syndesmotic DRUJ of most animals, the sigmoid notch of the distal radius forms a synovial joint with the ulnar head. Another major modification is that the distal end of the ulna no longer articulates with the triquetrum and the pisiform, and is reduced in size significantly to became the ulnar styloid process. Thus the human wrist has wider deviation in the coronal (radioulnar) plane and sufficient forearm rotation.

24.2 The Extensor Retinaculum

After removing the skin and subcutaneous tissue around the wrist joint, the extensor retinaculum, consisting of a supratendinous layer and an infratendinous layer, can be observed as a thickening of the antebrachial fascia. (▶Fig. 24.2). ¹¹ The fibers of the extensor retinaculum arise from the radiopalmar edge of the distal radius, run ulnarly parallel to the fibers of dorsal radiotriquetral ligament, skim the ulnar styloid distally, and end on the ulnar aspect of the pisiform. There are six dorsal compartments of the extensor retinaculum for the extensor tendons, divided by five septae. The 5th dorsal compartment exists between the supratendinous layer and the infratendinous layer of the extensor retinaculum that covers the dorsoradial portion of the DRUJ. The ECU tendon has its own synovial sheath that attaches on the dorsal aspect of the ulnar head under the extensor retinaculum to balance stability of the ECU and mobility of the DRUJ during forearm rotation. The ECU tendon sheath is considered a duplication of infratendinous layer of the extensor retinaculum (▶Fig. 24.3).

The posterior (dorsal) groove on the ulnar head and the posterior aspect of the ulnar styloid process are the place for the ECU tendon sheath to attach. The tendon sheath is a structure that consists of a tubular membranous body and a relatively thick fibrous floor, formed between the supra- and infratendinous extensor retinaculum. This structure can be divided into three parts. In the proximal portion, where the floor of the sheath strongly attaches on the ulnar head, the exterior of the sheath is covered by only thin antebrachial fascia. In the middle portion, between the ulnar head and triquetrum, the floor attaches on the dorsal aspect of the ulnar collateral ligament. Though the exterior of the membranous wall is covered with thick supratendinous retinaculum, these two are separated by loose connective tissue and the ECU tendon sheath is permitted a radioulnar translation under the retinaculum. In the distal portion dorsal to the triquetrum, the floor unites with the infratendinous retinaculum and attaches strongly on to the triquetrum. The membranous sheath also merges with the supratendinous retinaculum and permits little movement of the ECU tendon. Removal of both layers of extensor retinaculum except the ECU tendon sheath between the 5th dorsal compartment and the pisiform allows better observation of the TFCC (▶Fig. 24.4).

24.3 The Triangular Fibrocartilage Complex (TFCC)

The TFCC, as a term to indicate the ligamentous complex on the ulnar side of the wrist, was introduced by Palmer and Warner in 1981. ¹² It was a structural concept to define the classification of the injuries within this complex. They selected following six structures that have close relation in anatomy as structural components of TFCC: (1) articular disc, (2) meniscus homologue, (3) radioulnar ligaments, (4) palmar ulnocarpal ligaments, (5) ulnar collateral ligament, and (6) ECU tendon sheath (▶Fig. 24.5). The articular disc, or the “triangular fibrocartilage,” is located ulnarly, continuing from the sigmoid notch of the radius (▶Fig. 24.6). It has two smooth concave facets articulating with the ulnar head proximally and the carpal bones distally. The meniscus homologue, named to indicate the “homologous structure of ape meniscus in the human wrist joint,” is observed protruding from the dorsal and palmar joint capsule. ^{9 , 13} At the ulnarmost corner of the articular disc, between the dorsal and palmar lamellae of meniscus homologue, is a dimple named the “prestyloid recess,” the cavity of which is usually occupied by synovial villi. Normally, there is no communication between DRUJ and radiocarpal joint cavity unless there is tear or perforation of the TFC or disruption of TFCC.

The radioulnar ligaments, the palmar ulnocarpal ligaments, the ulnar collateral ligament, and the ECU tendon sheath are considered stabilizing structures of TFCC that support the articular disc and the meniscus homologue. The radioulnar ligaments that connect distal radius and ulna are observed at the dorsal and palmar portions of the articular disc after removing loose connective tissue of joint capsule and other external components of TFCC (▶Fig. 24.7 and ▶Fig. 24.8). The dorsal fascicle of the radioulnar ligament arises from the dorsal rim of the sigmoid notch and runs ulnarly uniting with the dorsal margin of the articular disc. The palmar fascicle arises from the palmar rim of the sigmoid notch and widely from the palmar rim of the lunate facet of the radius. These two fascicles merge at the ulnar apex of the articular disc, then divide into proximal (deep) and distal (superficial) layers, attaching on the fovea of the ulnar head and the ulnar styloid process, respectively (▶Fig. 24.9). This arrangement of the ligaments permits swing motion of the radius around the ulna (▶Fig. 24.10).

The palmar ulnocarpal ligaments connect the ulnar head and the carpus (▶Fig. 24.11). They include three ligament fascicles: (1) the ulnolunate ligament that connects the ulna and the palmar aspect of the lunate, proximal to the attachment of the lunotriquetral ligament; (2) the ulnotriquetral ligament that connects the ulna and the palmar aspect of the triquetrum, proximal to the pisotriquetral joint facet; and (3) the ulnocapitate ligament that connects the ulna and the palmar aspect of the capitate along with the fibers of triquetrocapitate ligament. The ulnocapitate ligament was clearly confirmed by gross anatomical dissections (▶Fig. 24.12). The proximal portion of the ulnocarpal ligaments interdigitate with the fibers of tongue like “distal extension” of the palmar radioulnar ligament (Palmar Distal Extension, PDE) (the term was suggested by Julio Taleisnik, MD in a personal communication) and attach on the base of the ulnar styloid process. The “distal extension” covers the palmar aspect of the short radiolunate ligament that connects the palmar rim of the radius and palmar proximal portion of the lunate, and its most distal portion loosely attaches on the palmar aspect of the lunate (▶Fig. 24.13). The ulnar styloid process, a cone-shaped bony protrusion located on the ulnar head, provides space for several structural elements of the TFCC to attach (▶Fig. 24.14). When the DRUJ is observed in neutral position of the forearm, the ulnar collateral ligament is seen to attach to the ulnar styloid proximally and the triquetrum distally. The distal (superficial) radioulnar ligament attaches on the radial (medial) aspect of the ulnar styloid process. The “distal extensions” of the palmar radioulnar ligament and the palmar ulnocarpal ligaments attach on the proximal portion of the radial aspect of the styloid process. In other words, the “distal extensions” of palmar radioulnar ligament, the palmar ulnocarpal ligaments, and the ulnar collateral ligament surround the distal (superficial) radioulnar ligament and attach together on the broad area of the ulnar styloid process. There exists a cavity surrounded by these ligaments that connects the radiocarpal joint and the tip of the ulnar styloid process. This cavity corresponds to the prestyloid recess (▶Fig. 24.6). The volar wall of the recess has a thin membranous portion between the ulnotriquetral ligament and the ulnar collateral ligament. This was recognized as an oval window, the result of anatomical dissection of the joint capsule, to remove as much areolar tissue and visualize the hidden capsular ligaments (▶Fig. 24.12). These findings suggested that the ulnar collateral ligament that connects the ulnar styloid process and the triquetrum can be recognized as a part of the ulnotriquetral ligament that consists of two fascicles separated by the anterior thin membranous portion. The ulnar insertions of the “distal extension” of palmar radioulnar ligament, the palmar ulnocarpal ligaments, the ulnar collateral ligament, and the distal (superficial) radioulnar ligament (DRUL) are separated from that of the proximal (deep) radioulnar ligament (PRUL) by vascular loose connective tissue. ^{9 , 14 , 21} There may exist variations in size of the opening space occupied by this tissue (▶Fig. 24.15). In some specimens, the opening is widely spread and the two ligament insertions are definite. Other specimens show a narrow opening where the insertions are in close approximation. These differences might be affected by the variations of this part, i.e., the thickness of the vascular areolar tissue, the length of the ulnar styloid process, or the ulnar variance during the development of DRUJ.